In the USA this symbol refers to "pound" because in telephone key boards it is called pound sign. In Brazil it is called "jogo da velha" (something like "old lady's game").
The most traditional name, in USA at least, is "number sign". In the past it was used only in the sense of number, so #1 means "number one".
In Portuguese this sign has three names officially: cerquilha, antifen e cardinal.
Is it becoming a bit confusing? Wait a minute because there is more about it.
In United Kingdom(England,North Ireland,Scotland and Wales) this sign has been always known as "hash" and from that came hashtag, meaning hash followed by a word that identifies something on the web.
When you write #something, it becomes a hyperlink making it possible for search engines to find that "something" on the web.
Hashtags must be short, isolated and mean the subject as clearly as possible, to avoid misunderstandings and false results in a search.
Tags are keywords connected to a subject we want to find on the web.

When the Loolapalooza festival happened in Brazil, people created three different hashtags: The official tag was #LollaBR but many people were writing #LollapaloosaBR or #LoolapaloosaBrasil.
That made the contents of the site to spread in three different hashes. OK it worked. But it would work better if everybody just used the official tag ok? 

Hashtags can also be used as an addendum, usually a bit funny, to a comment.
But don't exaggerate. Too much hashtags make reading confusing and boring and your blog or site will lose followers easily.

Nos EUA este símbolo se refere a "libra" (leia Pesos e medidas) , porque no teclado dos telefones  é conhecido como "sinal libra". No Brasil é chamado "jogo da velha".
O nome mais tradicional, nos EUA pelo menos,é sinal de numero. No passado era usado apenas neste sentido de modo que #1 significava "numero 1".
Em portugues este sinal tem três nomes oficialmente: cerquilha, antifen e cardinal. 
Esta ficando meio confuso? Espere um pouco porque ainda tem mais.
No Reino Unido (Inglaterra, Irlanda do Norte, Escócia e Pais de Gales) este sinal sempre foi conhecido como "hash" e dai veio hashtag, significando hash seguido de uma palavra que identifica algo na internet(web).
Quando voce escreve #algumacoisa, isso se torna um hyperlink que possibilita aos motores de busca acharem aquela "algumacoisa" na web.
Hashtags devem ser curtos, isolados e significar o assunto o mais claro possível, pra evitar mal entendidos e resultados falsos em uma busca.
Tags são palavras chave ligadas ao que queremos achar na web.